1. Field of the Invention
This invention relates to methods for classifying flaws present in an optical fiber.
2. Description of the Related Art
Optical fibers typically have a glass filament core, consisting of a cladding surrounding a wave guiding core, which in turn is surrounded by a protecting sheath of organic material. Commonly referred to as the "buffer layer", this sheath has been found to have flaws such as debris, bad splices, necks, nicks, bulges, bubbles, uncured portions or other non-uniformities which occur as a result of the fabrication and/or handling of the fiber. The existence of such flaws can become critical for certain fiber optic applications which place significant demands upon the fiber's tensile strength. One such stressful application is in the deployment of fiber during the flight of a fiber optic guided missile. In this situation, the fiber may be stressed at levels approaching its intrinsic strength. Pre-existing defects in the coating may permit mechanical contact with the glass fiber. This can weaken the fiber so that it is likely to break at stresses lower than the mechanical proof stress, which is estimated to be large enough to endure successful fiber payouts.
A detailed inspection of the fiber is thus important to ensure its survivability. This inspection has generally been performed manually with a microscope on the fiber end. However, with missile fiber lengths being on the order of tens of kilometers, manual inspection is inadequate. Furthermore, the inspection may have to be repeated to guarantee a rigorous control of fiber quality after handling and/or shipping.
An automated system for the detection of fiber flaws is disclosed in U.S. Pat. No. 4,924,087, issued May 8, 1990 in the name of Wilbur M. Bailey et al., "Optic Fiber Buffer Defect Detection System", assigned to Hughes Aircraft Company, the assignee of the present invention. In this patent a fiber travels past an inspection station where it is illuminated by one or more laser beams at right angles to the fiber axis. Scattering of the laser beams out of a plane orthogonal to the fiber axis is automatically observed and taken as an indication of a buffer flaw. The fiber can then be stopped and the flaw manually inspected to determine its nature, and whether it requires the flawed section of the fiber to be removed.
The improved system described above still requires a manual inspection step after the presence of a flaw has been detected. This step is both costly and time consuming. Furthermore, during the period the fiber is stopped for manual inspection, the automatic flaw detection system cannot be used for the remainder of the fiber.